Free Astronomy Magazine January-February 2016

SPACE CHRONICLES directly.) Careful analysis of the polarisation results revealed these grains of dust to be comparative- ly large particles, 0.5 micrometres across, which may seem small, but grains of this size are about 50 times larger than the dust normally found in interstellar space. Throughout their expansion, massive stars shed large amounts of material — every year, VY Canis Majoris sees 30 times the mass of the Earth expelled from its surface in the form of dust and gas. This cloud of material is pushed outwards before the star explodes, at which point some of the dust is destroyed, and the rest cast out into interstellar space. This material is then used, along with the heavier elements created during the super- nova explosion, by the next genera- tion of stars, which may make use of the material for planets. Until now, it had remained mys- terious how the material in these giant stars’ upper atmospheres is pushed away into space before the host explodes. The most likely driver has always seemed to be radiation pressure, the force that starlight ex- erts. As this pressure is very weak, the dust particles must be large enough to ensure the starlight can push it, but not so large that it sim- ply sinks. Too small and the starlight would effectively pass through the dust; too large and the dust would be too heavy to push. The dust the team observed about VY Canis Ma- joris was precisely the right size to be most effectively propelled out- wards by the starlight. “Massive stars live short lives,” says lead author of the study, Peter Scicluna, of the Academia Sinica In- stitute for Astronomy and Astro- physics, Taiwan. “When they near their final days, they lose a lot of mass. In the past, we could only theorise about how this happened. But now, with the new SPHERE data, we have found large grains of dust around this hypergiant. These are big enough to be pushed away by the star’s intense radia- tion pressure, which explains the star’s rapid mass loss.” The large grains of dust observed so close to the star mean that the cloud can effectively scatter the star’s visi- ble light and be pushed by the radia- tion pressure from the star. The size of the dust grains also means much of it is likely to survive the radiation produced by VY Canis Majoris’ inev- itable dramatic demise as a super- nova. This dust then contributes to the surrounding interstellar medium, feeding future generations of stars and encouraging them to form plan- ets. The star explosion will be soon by astronomical standards, but there is no cause for alarm, as this dramatic event is not likely for hundreds of thousands of years. It will be spec- tacular as seen from Earth — per- haps as bright as the Moon — but not a hazard to life here. T he star VY Canis Majoris is a red hypergiant, one of the largest known stars in the Milky Way. It is 30–40 times the mass of the Sun and 300,000 times more luminous. In its current state, the star would encompass the orbit of Jupiter, having expanded tremendously as it enters the final stages of its life. New observations of the star using the SPHERE instrument on the VLT have clearly revealed how the brilliant light of VY Canis Majoris lights up the clouds of material surrounding it and have allowed the properties of the com- ponent dust grains to be determined better than ever before. In this very close-up view from SPHERE the star itself is hidden behind an obscuring disc. The crosses are artefacts due to features in the instrument. [ESO] n